Setting the WithStart() and/or WithEnd() parameter will always cause an error. ("The request parameter validation failed. start is invalid format.")
For example modifying DotNet_Example 2.0.0:
response = HistoricalPricing.GetSummaries(new HistoricalPricingSummaries.Params().Universe(ric)
.WithInterval(HistoricalPricingSummaries.Interval.P1D)
.WithStart(new DateTime(1973, 4, 13))
.WithCount(2));